The invention relates to a gas generator, in particular for a vehicle occupant restraint system.
A gas generator known from the WO 97/41007 has a filter disc and a combustion chamber filter arranged upstream of a wall which delimits the filter chamber. The filter disc is constructed elastically, in order to hold the solid propellant, provided in tablet form, securely in place in the combustion chamber.
In the case of very long combustion chambers, in particular in combustion chambers in so-called tubular gas generators in which the gas generator has an elongated cylindrical housing, non-reproducible and too high combustion chamber pressures may occur. Furthermore, the filter units were acted upon too intensively with particles which arise on combustion of the propellant. For this reason, the filters became partially clogged. A solution to the said problem was to be provided by thin filters which were accommodated transversely to the main flow direction of the generated gas in the combustion chamber and which divide the combustion chamber into two partial chambers. However, here also there is a need for improvement, because in particular in combustion chambers which are filled with a large quantity of propellant, combustion chamber pressures which were too high were reached in the central region between the igniter unit and the outflow end of the combustion chamber. In this region, it is supposed that there occurs a particularly unfavorable configuration of extremely high flow speed of the generated hot gas and a high mechanical stress of the solid propellant, which is not yet fully burnt, as a result of being acted upon by hot particles. Mechanical damage (burning through) of the components (e.g. of the filter) can occur. Furthermore, the combustion chamber pressures may be too high and non-reproducible. A further disadvantage is the destruction of the solid propellant tablets, which are not yet burnt, through the high flow speed and the particles.
The invention provides a gas generator in which the above-mentioned disadvantages are avoided. This is achieved in a gas generator which comprises a housing, a filter chamber, and a combustion chamber which is filled with solid propellant and which has an outflow end. The gas generator further comprises at least one igniter unit which has an igniter and ignites the solid propellant, and comprises at least one filter which adjoins the solid propellant at the outflow end and delimits the combustion chamber. The filter has a central part which at least in sections has the shape of a cone and tapers in a direction towards an interior of the combustion chamber. The solid propellant laterally adjoins the conical section. By means of the conical, preferably cone-shaped central part, which brings about a central recess in the combustion chamber, the quantity of solid propellant in the region of the center of the combustion chamber, where the above-mentioned unfavorable configuration of high flow speed and high stress occurs owing to the particles, is reduced. The generated hot gas can flow out earlier from the combustion chamber via the conical filter and can laterally flow through the filter as it immediately laterally adjoins the conical section. A better reproducibility of the combustion chamber pressures results, as tests have shown. In addition, the filter is no longer damaged by the hot particles. Owing to the conically tapering central part, the filter has a larger contact surface area for the particles than, for example, a disc-shaped filter arranged in a cylindrical combustion chamber. The contact surface area is quite clearly in a ratio to the length of the combustion chamber (the length is related to the main flow direction). If the combustion chamber is very long, then the contact surface area of the filter must be larger than in the case of a very short combustion chamber, so that the above-mentioned disadvantages can be avoided. Hence, with a short combustion chamber, the filter delimiting the combustion chamber preferably has a cone with a smaller height than with a longer combustion chamber, in which the recess formed by the filter is to project deeper into the combustion chamber.
The filter with conical central part is suitable in particular for a tubular gas generator with cylindrical combustion chamber, because here particularly long flow paths occur.
The filter can have a central part in the shape of a truncated cone, a cylindrical section being provided which adjoins the central part in the direction towards the interior of the combustion chamber, so that as a whole a long filter is produced, providing the combustion chamber with a deep indentation.
The filter is preferably manufactured from a plurality of layers, e.g. from a plurality of layers of a metal screen or of an expanded metal.
Preferably, downstream of the combustion chamber a filter chamber is arranged, which has a wall provided with throughflow openings. Upstream of this wall, the filter is provided which spaces the combustion chamber from the filter wall. Between this filter and the wall of the combustion chamber, an expansion chamber for hot gas can be formed.
According to a preferred embodiment, at least one gas-permeable intermediate wall is arranged in the combustion chamber, the intermediate wall dividing the combustion chamber into at least two partial chambers which are filled with propellant. These partial chambers are open towards the igniter unit, the partial chambers each comprising an open end so that the propellant contained in the two partial chambers is ignited via the open ends. The intermediate wall is therefore at least largely not present towards the igniter unit, preferably is even omitted entirely, so that the propellant in the two partial chambers can be ignited in parallel in terms of time. Through the provision of this intermediate wall, the active contact surface area is once again increased. The use of the additional intermediate wall is advantageous in particular in combustion chambers which have a long flow path. The intermediate wall can establish a flow channel within the combustion chamber and at the same time can make available a surface against which particles can accumulate. Thereby, the conical screen is relieved.
The intermediate wall reduces the pressure in the combustion chamber, occurring through the hot gas, and prevents a destruction of the solid propellant tablets. The igniting of the propellant as a whole becomes more reproducible. The gas generator according to the invention, constructed in such a manner, provides for a rapid pressure rise in the connected consuming device, despite a reduction of the pressure in the combustion chamber.
The simultaneous igniting of the propellant in the two chambers can be facilitated in that the two partial chambers towards the igniter unit continue into a common connecting section, filled with propellant, of the combustion chamber. Firstly, the propellant is burned in the connecting section by the igniter unit. Then the propellant ignites in each partial chamber.
For this, the intermediate wall ends in front of the connecting chamber.
In order to reduce the stress of the intermediate wall by the hot particles, the intermediate wall is preferably arranged substantially parallel to the main flow direction within the combustion chamber. The main flow direction is defined by the form of the combustion chamber and in particular by the arrangement of the igniter unit and through-flow openings in the wall which closes off the filter chamber towards the combustion chamber.
A simple manufacture of the conical filter together with the intermediate wall and a simple assembly can be achieved in that the conical filter and the intermediate wall are fastened to each other.
The gas generator according to the invention, as already mentioned, is preferably a tubular gas generator with an elongated cylindrical housing. Here, the igniter unit is arranged at an axial end and the combustion chamber and the filter chamber are arranged axially one behind the other. The cone of the filter is arranged coaxially to the housing, therefore extends into the center of the combustion chamber. Preferably, the intermediate wall is also arranged coaxially to to the housing, i.e. parallel to the main flow direction, which runs axially.